You are mixing tabs and spaces. Don't do that. Specifically, the __init__ function body is indented with tabs while your on_data method is not.

Here is a screenshot of your code in my text editor; I set the tab stop to 8 spaces (which is what Python uses) and selected the text, which causes the editor to display tabs with continuous horizontal lines:

You have your editor set to expanding tabs to every fourth column instead, so the methods appear to line up.

Run your code with:

python -tt scriptname.py

and fix all errors that finds. Then configure your editor to use spaces only for indentation; a good editor will insert 4 spaces every time you use the TAB key.